This invention relates to a blade of a gas turbine having a novel heat-resistant ceramic coating layer, and also to a method of producing such a blade.
In a power-generating gas turbine, in order to enhance the power-generating efficiency, it is essential to achieve a high operating temperature and also to reduce the amount of a cooling medium, and it has been earnestly desired to enhance the high-temperature durability of turbine blades. In addition, in order to reduce the running cost of the gas turbine, it has been essential to prolong the service life of the turbine blades. Under the circumstances, heat-resistant materials, having high-temperature strength and excellent reliability, have been developed, but the heat resistance of the material itself is limited. Therefore, a thermal barrier coating (hereinafter often referred to as xe2x80x9cTBCxe2x80x9d) has been proposed in order to reduce the substrate metal temperature of the blade used under high temperature conditions, and by using the TBC in combination with the cooling of the blades, the substrate metal temperature can be made 50 to 200xc2x0 C. lower than that achieved without the TBC.
However, the TBC, used under severe thermal load conditions, is susceptible to damage such as the separation of a ceramic layer, and this damage is conspicuous particularly in a high-temperature gas turbine designed to achieve the enhanced power-generating efficiency. Therefore, various TBCs, having further improved durability, have been proposed, and examples of such prior art techniques are as follows.
(1) U.S. Pat. No. 4,503,130
In these prior art techniques, there are disclosed a TBC of a columnar structure having a columnar-crystal ceramic layer having a thermal stress relaxation function, and a TBC of a porous structure having a porous ceramic layer, and damage due to the thermal expansion difference between the ceramic layer and heat-resistant alloy constituting the substrate, is prevented.
More specifically, the ceramic layer is composed of a ZrO2 ceramic material which has low thermal conductivity and high thermal expansion, and has excellent stability at high temperature. In this case, when ZrO2 is used alone, damage due to a dimensional change, caused by phase transformation, is encountered, and therefore stabilized ZrO2, which is prevented from phase transformation by the addition of a crystal structure-stabilizing agent (such as Y2O3, CaO, MgO, CeO2, Sc2O3), or partially-stabilized ZrO2 is used. With respect to the structure of the ceramic layer, the columnar structure or the porous structure is used. The TBC comprises the ceramic layer and a primary coat (underlayer) for this ceramic layer, and as the primary coat or layer, there is used MCrAlY alloy (M is one of Co, Ni, Fe or a combination thereof), having excellent high temperature oxidation resistance and corrosion resistance, or MCrAlY alloy whose surface portion is made rich in Al. In one example, the surface of the primary layer (i.e., underlayer) is beforehand oxidized to form a thin layer of Al2O3.
In the case of using such TBC in a blade of a high-temperature gas turbine, the TBC is applied to blades having a cooling function, so as to achieve a thermal barrier effect. More specifically, the temperature of combustion gas is high on the blades of the high-temperature gas turbine, and therefore the cooling performance of the blades is high, and the quantity of heat (heat flux), penetrating the blade, becomes large. As a result, in the TBC provided on the surface of the blade, the thermal barrier effect is enhanced because of the large heat flux, and the effect of the TBC is enhanced. As the heat flux increases, the temperature of the ceramic layer, constituting the outermost layer of the TBC, rises. As a result, the oxygen ion conductivity of the ZrO, ceramic, which is its inherent property, becomes higher, and the supply of oxygen to the surface of the primary layer (underlayer) through the ceramic layer increases, so that an oxidation reaction at the surface of the primary layer is promoted. Here, the primary layer comprises the MCrAlY alloy layer, and this alloy layer has the function of preventing damage due to the oxidation reaction. Particularly at high temperatures, Al, contained in the alloy, is oxidized into Al2O3, and this Al2O3 forms the barrier layer to prevent the internal oxidation of the alloy itself. Incidentally, ceramic members, having a TBC similar in structure to the above TBC, are disclosed, for example, in Japanese Patent Unexamined Publication Nos. 6-256926 and 8-27559.
(2) Japanese Patent Unexamined Publication No. 5-279832
In this prior art technique, a MCrAlY alloy layer is formed on a substrate, and a porous ceramic layer of a molten particle laminate structure is formed by spraying on this alloy layer, and pores in this ceramic layer are filled with Al2O3, and by doing so, the internal destruction to thermal stress relaxation is prevented.
(3) Japanese Patent Unexamined Publication No. 8-20878
In this prior art technique, a MCrAlY alloy layer is formed on a substrate, and a porous ceramic layer of a molten particle laminate structure formed by spraying, or a ceramic layer of a columnar structure formed by electron beams is formed on this alloy layer, and a thin coating of Al2O3 is formed on the surface of the ceramic layer, and by doing so, the oxygen permeability in the ceramic layer is lowered, thereby enhancing the durability.
However, the above prior art techniques have the following problems.
In the TBC of the prior art technique (1) and the ceramic member having this TBC, the Al2O3 barrier layer is formed by Al contained in the MCrAlY alloy layer, and therefore under conditions in which the oxidation is promoted at high temperatures, the thickness of the Al2O3 layer is increased by Al of the Al2O3 layer or by diffusion of oxygen. Therefore, for example, when this material is used in a high-efficiency power-generating gas turbine having high operation temperature, since there occur severe thermal load conditions in which the starting operation, the long-time holding of the steady state, and the stop are repeated, damage is caused in the vicinity of the Al2O3 layer by thermal stresses of the heat cycle due to the increase in thickness of the Al2O3 layer, and the separation of the ceramic layer is liable to occur in the vicinity of the boundary between the ceramic layer and the primary layer (Al2O3 layer), which leads to a fear that the thermal barrier effect, which is the originally-intended purpose, is not satisfactorily achieved. Namely, in the prior art technique (1), no consideration is given to the prevention of the thickening of the Al2O3 layer due to the increased oxygen-ion-conductivity of the ZrO2 ceramic layer at high temperatures so as to suppress the separation of the-ceramic layer.
Incidentally, it has been reported that such damage, developing in the vicinity of the Al2O3 layer formed at the interface of the primary layer and the ceramic layer, can also occur in a TBC in which a ceramic layer is of a columnar structure and which has a thermal stress relaxation function (Report of ASME meeting, 1991-GT-40). Further, it has been reported that in a porous ceramic layer formed by plasma spraying, a thickened Al2O3 layer at the interface determines the lifetime (durability) of a TBC (xe2x80x9cStudy of Estimated lifetime of Gas Turbine Barrier Coatingxe2x80x9d in Research Report vol. 36, No. 3, p475 by No. 123 Small Committee Research Report of Heat Resistant Metallic Materials; 1996).
In the prior art technique (2), as described above for the prior art technique (1), no consideration is given to the prevention of the thickening of the Al2O3 layer due to the increased oxygen-ion-conductivity of the ZrO2 ceramic layer at high temperatures so as to suppress the separation of the ceramic layer. It may be thought that the structure, in which the pores in the ceramic layer, are filled with Al2O3 for the purpose of preventing the internal destruction due to thermal stress relaxation, somewhat suppresses the thickening of the Al2O3 layer due to the increased oxygen-ion-conductivity. However, since only the pores are filled with Al2O3, its effect is insufficient. This prior art publication does not disclose the thickening of the Al2O3 layer which thickening is caused by the enhanced oxygen ion conductivity of the porous ceramic layer.
In the prior art technique (3), in both of the porous ceramic layer of a molten particle laminate structure and the ceramic layer of a columnar structure, consideration is given to the prevention of the increase in the oxygen-ion-conductivity of the ZrO2 ceramic layer at high temperatures. However, when this coating layer is used for a blade of a gas turbine, a lot of dust, including iron rust contained in combustion gas, flows and impinges on the coating layer (this is known as xe2x80x9cerosionxe2x80x9d), and therefore the Al2O3 layer on the surface thereof is scraped off by this dust, so that the satisfactory effect can not be obtained for a long period of time.
It is an object of this invention to provide a ceramic-coated blade of a gas turbine in which the thickening of an Al2O3 layer at the interface of a ceramic layer and a primary layer is sufficiently prevented for a long period of time, thereby positively suppressing the separation of the ceramic layer.
Another object of the invention is to provide a method of producing such a ceramic-coated blade.
According to the present invention, there is provided a ceramic-coated blade of a gas turbine and a method of producing the same, wherein the blade comprises: a substrate made of a heat-resistant alloy comprising Ni as a main component; an alloy coating layer which is formed on a surface of the substrate and which is made of a material superior in high-temperature oxidation and corrosion resistance to the heat-resistant alloy; and a heat-resistant ceramic layer formed on the alloy coating layer, wherein the heat-resistant ceramic layer is a mixture of ion-conductive ceramic, composed of at least partially-stabilized ZrO2 ceramic, and insulative ceramic; and the mixture has a columnar structure in which columnar crystals are grown in a direction of a thickness of the coating, or a porous structure in which breadths of flattened particles are laminated in the direction of the thickness of the coating, and the former is formed by a gas phase, and in the latter, molten particles are caused to fly at high velocity so as to form the latter structure.
The heat-resistant ceramic layer of the present invention has the porous structure formed by a molten phase produced by plasma spraying or the like, or the columnar structure formed by a gas phase produced by electron beam vapor deposition or the like.
(1) Porous Structure
In this case, the heat-resistant ceramic layer has the porous structure in which the flattened particles in the direction of the thickness of the coating are laminated, with residual pores intervening there-between. Conventionally, in this structure, oxygen intrudes through defects of the porous ceramic layer, and also oxygen is supplied to the interface of the heat-resistant ceramic layer and the primary layer because of the oxygen ion conductivity of the ZrO2 ceramic, so that Al, contained in MCrAlY alloy constituting the primary layer, is changed into Al2O3 by an oxidation reaction at the interface.
In the present invention, however, the heat-resistant ceramic layer comprises a mixture formed by adding insulative ceramic, such as Al2O3, to ZrO2 ceramic, and therefore the oxygen ion conductivity of the heat-resistant ceramic layer is greatly lowered, and hence the oxidation reaction at the interface is markedly suppressed. As a result, even when the blade is used under high-temperature conditions for a long period of time, the thickening of the Al2O3 layer, formed at the interface of the ceramic layer and the primary layer, is prevented. In this case, in the present invention, the pores in the porous structure remain, and the porous structure is formed by the mixture of the ZrO2 ceramic and the insulative ceramic, with the pores intervening therebetween. Namely, since the insulative ceramic is filled uniformly in all portions except the pores, the marked effect of reducing the oxygen ion conductivity can be obtained.
The morphology of presence of the insulative ceramic in the heat-resistant ceramic layer of such a porous structure may be one in which each of the flattened particles is in the form of mixture, or one in which the flattened particles of ZrO2 ceramic and the flattened particles of Al2O3 or the like are mixed and laminated. In the case where the individual flattened particle is formed by the mixture, the oxygen ion conductivity of the individual particles is lowered. On the other hand, in the case of the structure in which the ZrO2 particles and the Al2O3 particles are mixed together, the oxygen ion conductivity is lowered through the laminated particles. Of course, even with a structure in which the two mixture states are combined together, the effect of reducing the oxygen ion conductivity can be obtained.
Examples of methods of forming the ceramic layer, in which the ZrO2 ceramic and the insulative ceramic are mixed together, include a method in which a raw material of powder particles in a mixed state is sprayed, another method in which a powder mixture of ZrO2 and Al2O3 or the like is sprayed as raw material, still another method in which Al2O3 or a metallic element constituent (Al) of Al2O3 is coated on a surface of each ZrO2 particle, and these composite powder particles are sprayed as a raw material, and still another method in which the powder particles, combined together by the above methods, are sprayed as a raw material.
(2) Columnar Structure
In this case, the heat-resistant ceramic layer has the columnar structure in which the columnar crystals are grown in the direction of the thickness of the coating. Generally, conventionally, in this structure, since no particle boundary acting as barrier to oxygen ion conduction exists, the oxidation reaction, occurring at the interface of the heat-resistant ceramic layer and the primary layer by the oxygen ion conductivity as described above for the structure (1), tends to become very large in comparison with the porous structure in which the flattened particles are laminated.
In the present invention, however, the heat-resistant ceramic layer comprises the mixture formed by adding the insulative ceramic, such as Al2O3, to ZrO2 ceramic, and therefore the oxygen ion conductivity of the heat-resistant ceramic layer of the columnar structure is greatly lowered, and hence the effect of suppressing the oxidation reaction at the interface is greatly enhanced. As a result, even when the blade is used under high-temperature conditions for a long period of time, the thickening of the Al2O3 layer, formed at the interface of the ceramic layer and the primary layer, is prevented.
Examples of methods of forming such a heat-resistant ceramic layer include one in which ZrO2 ceramic and the insulative ceramic are simultaneously vapor deposited, and one in which the two ceramics are vapor deposited alternately. In either of the two methods, the heat-resistant ceramic layer, constituting the columnar structure, can be formed into a structure in which the ZrO2 ceramic and the insulative ceramic (e.g. Al2O3) are mixed together, and even at a nano order level, the two can be mixed generally uniformly, or can be alternately laminated in the form of crystals.
Preferably, the amount of the insulative ceramic contained in the heat-resistant ceramic layer is 5 to 50 wt. %.
The range of the insulative ceramic content of the heat-resistant ceramic layer of the present invention should fundamentally be determined by the oxygen ion conductivity of the ZrO2 ceramic at high temperatures. However, it is the most significant that the oxygen ion conductivity-reducing effect of the present invention should be determined upon studying the thickness of the Al2O3 layer at the interface of the heat-resistant ceramic layer and the primary layer in the TBC when it is heated at high temperature for a long period of time. Therefore, the optimum range of the amount of addition of the insulative ceramic can be determined by relation between the thickness of the Al2O3 layer at the interface in the TBC and the amount of addition of the insulative ceramic. Based on this concept, the 5 to 50 wt. % insulative material content is effective for preventing the thickening of the Al2O3 layer at the interface, and further with the 10 to 40 wt. % content, its effect is the most effective. One reason for this is that if the addition amount is less than 5 wt. %, the oxygen ion conductivity-reducing effect is low, and as a result the effect of suppressing an oxidation at the interface is low, so that the thickening of the Al2O3 layer can not be sufficiently prevented. Another reason is that if the content is more than 50 wt. %, the oxygen ion conductivity-reducing effect by the insulative ceramic becomes generally constant, and is not much lowered, whereas the thermal physical properties (thermal conductivity, thermal expansion, specific heat, irradiation and etc.), inherent to the insulative ceramic, become conspicuous as thermal characteristics of the ceramic layer, so that the thermal physical properties, inherent to the ZrO2 ceramic, are varied, thereby adversely affecting excellent thermal characteristics of the ceramic for a TBC. Therefore, preferably, the amount of addition of the insulative ceramic of the TBC ceramic of the present invention is in the range of from 5 wt. % to 50 wt. %.
Preferably, the insulative ceramic contains at least one of Al2O3, SiO2, HfO2, BN, AlN and Si3N4.
Preferably, the ion-conductive ceramic contains ZrO2 as a main component, and further contains at least one of Y2O3, CaO, CeO2 and MgO, and preferably the amount of the at least one substance is 4 to 12 wt. %, and more preferably 6 to 10 wt. %.